1. Field of Invention
The present invention relates to electrical components, and more particularly to electrical components, generally for thermal systems, molded within a polymer composite.
2. Description of Related Art
Electric resistance heaters are common place in industry, and generally comprise a resistance wire, through which an electric current is passed, a ceramic core, around which the same wire is disposed, a dielectric ceramic layer, which surrounds the current-carrying core, and a metal alloy sheath to complete the assembly. One form of electric resistance heater, known as a cartridge heater, which is used in a very wide range of applications, has a cylindrical sheath, which has historically been made of corrosion-resistant metal alloys such as stainless steel or incoloy. To enhance thermal performance of the heating element, the above assembly is typically swaged.
More recently, industry has been looking for alternative cartridge heaters that weigh less, cost less to produce, that can be designed with greater geometric flexibility, and that can be cost-effectively mass produced while yielding excellent thermal and mechanical performance. One solution was proposed in U.S. Pat. No. 5,586,214 to Eckman and jointly assigned to Energy Converters, Inc. of Dallas, Pennsylvania and Rheem Mfg. Co. of New York, N.Y. Eckman discloses an immersion heater, somewhat similar to a cartridge heater in shape, but being hollow and having apertures in the sheath. Instead of being a solid cylinder, the core represents an injection molded polymeric hollow tube onto which a sheath is injection molded. Therefore, the heater does not have a xe2x80x9ccorexe2x80x9d in the traditional sense. The Eckman heater is shown in FIG. 1.
The Eckman heater does have certain advantages over the prior art, such as low weight, low manufacturing cost at high volume, and its high resistance to galvanic corrosion and mineral depositing. Yet the Eckman heater has many limitations which leaves it undesirable for most applications other than low temperature and low heat flux water heating tanks.
This is supported by the limitation of thermoplastic matrices to accept filler medium. In this context, Eckman discloses that the filler level in these polymeric matrices cannot exceed 40% by weight, which correlates with the research results obtained during the development of the present invention.
Providing a solid core (or at least one of substantially greater wall thickness) in the Eckman heater is not as easy as changing the geometry of the polymer, around which the resistance wire is wound. If a core polymer with the same temperature dependent thermal expansion function as the outer polymer is used, the heater will be prone to cracking and failure when energized and brought to operating temperature. Eckman teaches that the outer polymer coating needs to be less than 0.5 inches and ideally less than 0.1 inches, which further sacrifices structural strength. Eckman achieves somewhat higher thermal conductivity and higher possible heat fluxes than would be found in a pure polymer by suggesting the use of carbon, graphite, and metal powder or flakes as an additive. The amount of these additives must be limited though to protect the heater""s dielectric strength. Even then, thermal conductivity does not get significantly better than 1.0 W/(m*K).
It is also preferable to have other components of thermal systems, such as sensors, control systems, and thermoelectric modules that can be used for either cooling or heating purposes, placed in a low weight, low cost housing. Most of these components however are not suitable or only marginally suitable for molding within an injection molded polymer. This is due, in part, to the high temperatures to which the parts are exposed during molding, and also in part due to the marginal rewetting capabilities of many injection molded plastics. Even with components that are not as vulnerable to the high temperatures of injection molding, such as some temperature sensors, the low thermal conductivity of these polymers, as mentioned above, limit the usefulness of the finished product.
It is thus an object of the present invention to provide a molded polymer composite heater with a composite filler level of substantially greater than 40%.
It is also an object of the present invention to provide a molded polymer composite heater with improved structural integrity.
It is further an object of the present invention to provide a molded polymer composite heater with greater core thickness up to the extreme where the hollow space in the center of the element vanishes.
It is yet another object of the present invention to provide a molded polymer composite heater with improved thermal performance, namely thermal conductivity and maximum heat flux.
It is still a further object of the present invention to provide a variety of electrical components for thermal systems encased in polymer sheaths.
Other objects of the invention will become apparent from the specification described herein below.
In accordance with the objects listed above, the present invention is a molded polymer composite heater having highly filled polymers, such that the polymers are best suited for either transfer molding or compression molding. Compared to the prior art, which specifically refers to injection molding, the present invention allows for much higher levels of fill. The higher levels of fill, which exceed 50% by weight and may reach as high as 90% by weight, provide polymer compounds with better mechanical properties such as strength and impact resistance, superior thermal properties, such as higher service temperatures, specific heat, and thermal conductivity, as well as improved electrical properties, such as dielectric strength and insulation resistance. The polymer composite core of the heater may have lead terminals inserted therein that contact an electrical resistance wire disposed therearound. It is also possible to mold the heater core and sheath at one time, in which instance there may not be a distinction possible between those two components.
The present invention also preferably uses a greater core and sheath thickness up to and including a solid core, which allows for a greater number of geometric variations and the possibility of including additional features in the heater. For instance, sensors may be included at a particular point in the heater, where temperature measurement is most critical, or microchips may be embedded within the heater providing controlling means integrated with the heater.
Thermoset polymers are preferably used, although a few select thermoplastics may be used as well. The polymers are filled with reinforcing additives, which increase viscosity of the raw and processable molding compound. For best results, the reinforcement level should exceed 50%. The structural integrity of thermoplastics diminishes quickly once reinforcement levels exceed 40%, thus the preference toward thermoset polymers which can exceed the 50% reinforcement level.
Different fillers may be used depending upon the particular need of an application. Some applications, will not need as much thermal conductivity, but will require high mechanical strength and impact resistance. Others may require high chemical resistance, low moisture absorption, etc.
The reinforcement filler may be made from a great number of materials, however many applications require good thermal conductivity of the polymer sheath. For such applications, it has been found that ceramic particulate or ceramic whisker fillers, such as magnesium oxide or boron nitride work well, in addition to many forms of carbon. One must be cautious in using carbon reinforcement, because it decreases the dielectric strength of the sheath and core. The present invention incorporates techniques that allow high fill levels (at least 60%) of carbon fibers without significant loss of dielectric strength, but provide good thermal conductivity and excellent mechanical strength.
According to one aspect of the present invention, the solid core is made of a polymer composite, as described above, formed into two interlocking halves. The halves may be made from the same mold, and have a self-mating feature, thus reducing the cost of manufacture.
The complete core will have bores for two or more pins. For power lead pins, the core will have sections that expose the bores, so that a resistance wire may be welded to the pins. Preferably, one exposed point of the power lead pins will be toward an end of the heater distal to where the lead pins emerge from the heater itself. Another exposed point should be proximate to the end where the lead pins emerge from the heater. This allows for a single wound resistance wire, which is desirable over looped (double wound) resistance wires that are more prone to high-potential short circuits.
Over the core, a polymer sheath is added. The sheath is primarily made of the same polymer composite as the core, although the exact composition may vary, particularly when differing coefficients of thermal expansion are desired, for high temperature applications (xcx9c greater than 300xc2x0 F.). Most of the sheath is added by transfer or compression molding. However, for applications requiring a high dielectric strength, an additional thin layer of polymer may be added by dipping, spraying, or screen printing, either the assembled core or the sheathed heater.
In producing the entire heater in a single mold, the heating element is inserted into a sandwich of sheet molding compound (SMC) or preformed bulk molding compound (BMC). The resulting sandwich is then placed in the compression mold for curing. This embodiment may not have a definable core.
The same technology can be applied to an unlimited number of electrical components, and is especially useful for components related to thermal systems. The resistance heating element can be replaced with two wires of different types wire in a any combination well-known in the art for making thermocouples or thermoelectric chillers.
In another embodiment, printed circuit boards with or without integrated circuits may be placed in a premolded polymer composite cradle. Thereafter, the circuit board and the cradle in which it rests are then encapsulated in a polymer shell applied preferably by over-molding. For example, integrated control systems may be placed in the polymer sheath together with a heater and internal temperature sensor.